Representative Todd Akin of Missouri in May when he announced his candidacy for the Senate.Credit
Jeff Roberson/Associated Press

WASHINGTON — Representative Todd Akin on Tuesday ignored a deadline to abandon Missouri’s Senate race and vowed to remain the Republican nominee in defiance of his party’s leaders, including the presidential standard-bearer, Mitt Romney.

“I believe the defense of the unborn and a deep respect for life, which underlie all of America, those are important parts of who we are. And they’re not things to run away from,” Mr. Akin said on the radio program hosted by Mike Huckabee, the former Arkansas governor.

Mr. Akin touched off a firestorm of criticism after he said in a television interview on Sunday that in instances of what he called “legitimate rape,” women’s bodies somehow blocked an unwanted pregnancy.

The congressman’s decision to remain in the race against Claire McCaskill, the incumbent Democratic senator, came the same day the Republican Party’s platform committee endorsed a constitutional ban on abortion that does not include an explicit exception for rape.

Less than a week before the national party gathers in Tampa, Fla., to formally nominate Mr. Romney as its presidential candidate, the national conversation has turned sharply to the kind of social issues the party has tried to avoid.

Mr. Akin is no stranger to incendiary comments. “We can’t run from our shadows every time someone says ‘abortion,’ ” he told Dana Loesch, a conservative talk radio host, on Tuesday.

For months, Mr. Romney has struggled to stay focused on the economy while trying to narrow a deficit that polls show he has with women in the presidential race. But the week’s events have set back that effort and ensured a media spotlight for Mr. Akin and his ardent supporters in the social conservative movement.

The Akin showdown has raised fears among Republicans that they could fall short of winning control of the Senate and has magnified the schism within the party between activists nationwide still driven by core conservative values and party leaders in Washington, who are wooing independents and Democrats concerned about the struggling economy.

While virtually the entire party apparatus in Washington and a lineup of former Republican senators from Missouri turned on him, Mr. Akin rallied support among grass-roots voters, and he said that would propel him to victory.

Ione Dines, 72, from Marshfield, Mo., a Republican activist for 45 years and a Missouri state committee member since about 2006, said she was “just devastated” by her party’s actions.

“I am so disappointed in our national and our state committee when we throw our own under the bus,” she said.

Party leaders remained just as resolute in their demands for Mr. Akin to leave the race. Mr. Romney ramped up his appeal, saying, “Today, his fellow Missourians urged him to step aside, and I think he should accept their counsel and exit the Senate race.”

Both Karl Rove, co-founder of the deep-pocketed Republican group Crossroads GPS, and the National Republican Senatorial Committee made clear they would withhold support for his candidacy, even though a loss in Missouri could jeopardize any chance Republicans had for capturing the Senate.

“By staying in this race, Congressman Akin is putting at great risk many of the issues that he and others in the Republican Party are fighting for,” said Brian Walsh, a Senate campaign committee spokesman.

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The political repercussions of Mr. Akin’s decision are still unclear, but a race that Republicans need to win and thought they had sewn up had clearly shifted. The nonpartisan Cook Political Report on Tuesday flatly declared Mr. Akin “unelectable.”

But the Rothenberg Political Report, also nonpartisan, was more cautious, moving its evaluation of the race from tilting Republican to a pure tossup. The Rothenberg report had been prepared to declare the race firmly leaning Republican, had one of Mr. Akin’s rivals in the Aug. 7 primary, former state Treasurer Sarah Steelman and John Brunner, a businessman, won the nomination, said Stuart Rothenberg, the report’s editor.

Mr. Rothenberg said Mr. Akin would have trouble raising campaign money and appealing to swing voters. But “more than anything else, we thought, ‘We’re in the middle of firestorm.’ We want to wait to see what it looks like in a couple of weeks.”

Mr. Akin found support among anti-abortion activists and Christian conservatives, in Missouri and around the country. “Missouri Right to Life supports Congressman Akin’s defense of the life of an innocent unborn child conceived by rape,” declared Pam Fichter, president of the group’s political action committee.

“No one is speaking up on his behalf — this is a travesty,” said Rick Mathes, executive director of the Mission Gate Prison Ministry in Chesterfield, Mo., and an acquaintance of Mr. Akin’s. “With all this negative publicity, it’ll have a reverse spin, just watch. More people will be getting out to vote for him.”

In the radio interview with Mr. Huckabee, Mr. Akin cast himself as “Braveheart,” pilloried for “one word and one sentence on one day” by a cowardly establishment.

But for an increasing number of Republicans, that one word — “legitimate” — in that one sentence, “If it’s a legitimate rape, the female body has ways to try to shut that whole thing down,” was too much to bear.

Roy Blunt, Missouri’s Republican senator, spoke personally with Mr. Akin. Then when his appeals got nowhere, he helped organize a joint statement from himself and four former Missouri Republican senators: John Ashcroft, John C. Danforth, Christopher S. Bond and Jim Talent.

“We do not believe it serves the national interest for Congressman Todd Akin to stay in this race,” it said. “The issues at stake are too big, and this election is simply too important.”

More Republicans piled on, including several senators Mr. Akin hopes to join as colleagues: Senators Kelly Ayotte of New Hampshire, Richard Burr of North Carolina and Olympia J. Snowe of Maine all called for him to step aside.

But the national party’s public pressure campaign rankled some Republicans in Missouri. Violet Corbett, chair of the Johnson County Republican Central Committee and a member of the Republican State Committee, said she surveyed her county’s central committee on Monday night. The outcome: 31 people wanted Mr. Akin to leave the race; 2 wanted him to stay.

But by Tuesday, she had begun getting phone calls from fellow Republicans in the state who seemed put off by so much pressure from Washington. “They don’t know why we should be letting national tell us what to do here in Missouri,” she said.

Mr. Akin also received some support from his Democratic opponent, Ms. McCaskill, who used a round of advertisements in the last days of the Republican primary campaign to attack his opponents and boost his image.

Amid the calls for him to step aside, she told a St. Louis television station it would be a “radical thing” for the national party “to try to force someone who had won an election honestly off the ballot just because you think you want to pick another candidate.”

Mr. Akin released an advertisement asking the voters of Missouri to forgive him for saying “legitimate rape” does not lead to pregnancy. “Rape is an evil act,” the congressman said, speaking directly to the camera. “I used the wrong words in the wrong way, and for that I apologize.”

Republican Party officials had hoped to push Mr. Akin out by the first deadline, set by Missouri law, at 5 p.m. on Tuesday but it passed with him taking no action. The candidate can still take his name off the ballot up to Sept. 25, but the withdrawal could be contested by Missouri’s secretary of state, a Democrat, or any election authority in the state, even one at the city or county level. That is a fight Republicans want to avoid.

Monica Davey and Steven Yaccino contributed reporting from Chicago.

A version of this article appears in print on August 22, 2012, on page A1 of the New York edition with the headline: Ignoring Deadline to Quit, G.O.P. Senate Candidate Defies His Party Leaders: Akin Digs In and Defends Stand on Abortion. Order Reprints|Today's Paper|Subscribe